Regarding wrong value in credit memo req

Hi All,

We have created service contract with periodic billing(for each month) and net value calculated for 12 quantity for one year(12 months) contract and the value is for example(83 * 12) = 996. and we have created invoice for 996 value.

Now, when we are trying to create credit memo req with reference to the invoice, net value is changing to 1079(i.e 996 + 83). Net value appearing as one month amount more. that is 13 months.

Kindly help to undersatnd, why net value is appearing for 13 months in credit memo req, when referenced invoice value is for 12 months. will I have to change copy control settings to get copy correct value from invoice to credit memo req??